There are known crankshaft milling machines as a machine tool for cutting the pin part or journal part of crankshafts for use in an internal combustion engine. In a crankshaft milling machine, reciprocating tables (saddles) that reciprocate in a lengthwise direction of a bed are provided; slides are so supported as to be slidable in a direction perpendicular to the reciprocating direction of the reciprocating tables; and a spindle head with an inner-tooth milling cutter is supported on the slides such that it can rock or move up and down freely. In such a crankshaft milling machine, the inner-tooth milling cutter is set in circular motion through a combination of the rocking movement or vertical movement of the spindle head and the horizontal movement of the slides. The inner-tooth milling cutter in circular motion performs cutting operation on the pin part and journal part of the crankshaft fixedly supported at both ends thereof.
When a workpiece such as a crankshaft is cut by means of the above-described crankshaft milling machine, the thermal change of the machine itself occurs with the passage of time, causing such an undesirable situation that the sizes of a finished article exceed specified tolerances. In order to solve this problem, conventional milling machines are designed such that initial values for processing data are preliminarily set to the lower limits of the allowable ranges in case thermal displacement would occur, so that the sizes do not exceed upper limits even if thermal displacement of the machine body occurs. However, even though initial values are arranged as described above, the sizes of a finished article unavoidably exceed their tolerances in some cases. In these cases, the machine is stopped to manually input correction data in the NC device. Alternatively, processing data is recalculated whenever correction is carried out particularly in cases where there arises a need for changing the path followed by the inner-tooth milling cutter which performs circular motion. Manually inputting correction data or recalculation of processing data for every correction disadvantageously brings about a decrease in the efficiency of machining operation, since the machine has to be stopped for every correction or it takes a time for calculation.
The present invention has been made in order to overcome the foregoing problems and one of the objects of the invention is therefore to provide a system for controlling a crankshaft milling machine, which is capable of automatically, rapidly correcting processing data according to thermal displacement of a machine body to ensure high accuracy.